A Caesarean section increases the risk by 50-fold that a woman's uterus will rupture during a subsequent vaginal delivery, research suggests.

A torn uterus can put the life of both the mother and baby in danger.

US and Swedish researchers found the condition afflicted nine in every 1,000 mothers who opted to try for a vaginal birth after a previous Caesarean.

In contrast, the BJOG study found the rate among women with no history of a Caesarean was just 0.18 per 1,000.

Although uterine rupture is a relatively rare occurrence, the consequences can be devastating

Professor Philip SteerBJOG editor-in-chief

Many women who have previously had a Caesarean are offered the option of another, planned Caesarean second time around.

However, around a third of women opt to try for a vaginal birth to avoid what is a major operation which carries risk both for the mother and baby.

The findings were based on a study of more than 300,000 Swedish women by Emory University, Atlanta, and the Karolinska Institute in Stockholm.

Fourteen of the 274 women who suffered a torn uterus lost their baby - a death rate of 51 per 1,000.

In contrast, the neonatal death rate among women who did not develop the condition was just 1.4 per 1,000.

Other risk factors

A prior Caesarean section was not the only factor which increased risk.

Women who gave birth aged 35 or older were nearly three times more vulnerable to a uterine tear than women aged 24 or younger.

Clinically obese women had more than twice the risk of women who were not overweight.

And inducing labour appeared to double the risk, compared to labour which began spontaneously.

The researchers suggested the chemicals used to induce birth weakened previous Caesarean scars, making them more likely to rip.

Women who gave birth late were discovered to be more at risk than those who gave birth after a normal-length pregnancy, regardless of whether they had had a Caesarean section before.

And women who gave birth to babies weighing at least 4kg were at twice the risk than women whose babies were less than 4kg.

Researcher Dr Melissa Kaczmarczyk, of Emory University, said it was important that patients were made fully aware of the risks, and that those at higher risk were carefully managed through labour.

Professor Philip Steer, BJOG editor-in-chief, said: "The rate of Caesarean deliveries continues to increase in the developed world which means that a growing percentage of women will experience birth following a previous Caesarean section.

"Although uterine rupture is a relatively rare occurrence, the consequences can be devastating.

"The link between a prior Caesarean section and uterine rupture during subsequent delivery warrants very careful management of pregnancy and labour so that early signs of difficulty can be speedily detected."

Mervi Jokinen, of the Royal College of Midwives, said the study highlighted the fact that a Caesarean carried a long-term risk to health.

"We believe the Caesarean rate should be between 10% and 15%, but at present it is 23% in the UK," she said.